Since their development in the 1950's, the use of solid tantalum capacitors having high volumetric efficiency and reliability has increased to the current world market of several billion solid tantalum capacitors per year. The market continues to demand reliable capacitors with even higher volumetric efficiency and more restrictive parameters (e.g., lower equivalent series resistance, lower direct current leakage, etc.) at lower costs.
The electrochemical anodizing processes in use today for the manufacture of powder metallurgy tantalum capacitors typically employ electrolytes containing water, ethylene glycol or polyethylene glycol, and phosphoric acid. The resistivities of these electrolytes are usually excessively high for low voltage anodizing of anodes fabricated from high surface area metal powders (surface area is greater than 35,000 microfarad-volts/g) unless the phosphoric acid content exceeds approximately 1 vol. %. The relatively high percentage of phosphoric acid in protic acid solutions (i.e., water and glycols or polyglycols) results in relatively low pH values (e.g., pH less than about 2) and limited electrolyte life due to buildup of corrosion products in the electrolyte as well as the limited current carrying capability of the electrolyte due to precipitation of phosphate residues in the smaller pores of the powder metallurgy anode bodies fabricated from valve metal powders.
British patent G.B. 2 168 383 describes the use of amine phosphate salts, soluble both in organic solvents and water for use in anodizing electrolytes. Unfortunately, the amines which form soluble phosphate salts (e.g., tri-N-propyl amine, pyridine, and substituted pyridines) tend to be toxic and volatile at standard anodizing temperatures (from about 80.degree. to about 90.degree. C.). Additionally, these amines form incompletely ionized phosphate salts resulting in lower electrolyte conductivities than phosphoric acid alone.
U.S. Pat. No. 5,716,511 describes the use of phosphoric acid or acid phosphate salts in electrolytes containing up to 75 vol. % polyethylene glycol dimethyl ether for the anodizing of tantalum powder metallurgy anodes at temperatures below about 50.degree. C. in order to reduce the incidence of blister-like oxide flaws in higher voltage (above 100 anodizing volts) films formed over mechanically damaged tantalum surfaces.
As the surface area of the metal powder used for capacitor anodes has increased above about 0.35 square meters per gram (35,000 microfarad-volts per gram), the stabilization time required at voltage to obtain uniform oxide thickness has steadily increased due to voltage drop through the fine pores of the anode. Additionally, acidic phosphate-containing electrolytes leave increasingly obtrusive phosphate residues in the fine anode pores as the surface area of the powder has increased over the years. The required stabilization time at voltage and the accumulation of phosphate residues in the fine pores of anodes are further aggravated as the size of the anode bodies is increased.